I was researching tubes for my first designs.
THe 6SN7 is on almost everyone's favorites list.
However, I was having trouble determining if the EH version of the 6SN7 was a 300V or a 450V version.
An E-mail to New-Sensor only said they follow the RCA spec.

Also, Electro-Harmonix seems to be the only source of current 6SN7 tubes. While I have nothing against NOS, I feel more secure designing with tubes that are in production.

The 6SN7 is a Russian made tube currently and work well. All these Octal pin out devices suffer from hum injection when used with AC heaters due to the fact that the grid unit 2 is next to heater pin. When they planned the pin asignments back in the '30s they only thought of RF not audio and still used the same pinouts for the 6SL7 hi u tube.

Didn't know that about the pin assignments and hum. Last night I was testing an amplifier - hummed quite badly. Swapped out the 6SN7 front end tubes, and silence. Here's the kicker. The humming tubes were Sovtek, the quiet ones were a cheap no-name Chinese.
However I've never experienced any hum problems from EH tubes and they sound better.

Yes, there are variations in construction between various makers.
the differences in the lead outs to the base in capacity. The russians still use the original method of internal pinch( light bulb style) with sleaves over those that cross over between the pinch and the base. plus the small capacity between pins 8 and 1.
So in high input impedance circuits eg 47K there will be some hum
mesurable. So DC heaters are the order of the day.
Power amp phase spliters etc., are non critical.